[{_id:1,children:[{childrenOfChildren:[{price:10},{price:43},{price:55}]},{childrenOfChildren:[{price:2},{price:445},{price:3}]},{childrenOfChildren:[{price:45},{price:4},{price:2}]}]},{_id:2,children:[{childrenOfChildren:[{price:103},{price:43},{price:55}]},{childrenOfChildren:[{price:2},{price:34},{price:321}]},{childrenOfChildren:[{price:45},{price:4},{price:22}]}]}]

db.collection.aggregate([{$unwind:"$children"},{$unwind:"$children.childrenOfChildren"},{$group:{_id:null,maxPrice:{$max:"$children.childrenOfChildren.price"}}}])